Augmented Reality (AR) for interactive entertainment is exploring the potential of open space and multiplayer synchronization to expand user experience on mobile devices. However, complex interaction problems have hindered the development of related products. Based on a hybrid research methodology, this study proposes a set of design strategies including design elements, design framework, and functional attributes with priority ranking. To verify the effectiveness of the strategies, we developed ParallelWorld as an application example. It incorporates three basic modes of multiplayer-synchronized AR interaction: player to player, multiplayer to one virtual element, and multiplayer to physical space. Usability test and user interview suggest that ParallelWorld delivers a qualified experience for synchronized multiplayer in open space.